The mitochondrion is a semi-autonomous organelle that contains genetic information vital for its morphogenesis. During the past several years numerous cytoplasmic mutants of Saccharomyces cerevisiae have been isolated that are defective in components of the terminal respiratory pathway. These mutants define a new class of mitochondrial genes (mit genes) that are now known to code for subunit proteins of coenzyme QH2-cytochrome c reductase, cytochrome oxidase and the oligomycin-sensitive ATPase. The aims of this project are: 1) to saturate the mitochondrial genome with respect to structural and/or regulatory genes that are involved in the biosynthesis of cytochrome oxidase and coenzyme QH2-cytochrome c reductase, 2) to identify the products of the mit genes, and 3) to study the assembly of inner membrane enzymes by making use of nuclear and cytoplasmic mutants in each of the two respiratory complexes.